Superconductors are capable of transmitting electricity with virtually no resistance. Accordingly, in an effort to realize significant energy savings in the use and transport of electricity, there continues to be intense interest is using superconductors in the electric power grid.
All currently known superconductors require a significant degree of temperature lowering to achieve the zero-resistance state. Since the means for temperature lowering amounts to a significant expense, high temperature superconductor (HTS) materials have been of primary interest for commercial application. Some of the first HTS materials are the lanthanum barium copper oxides (LBCO) and the lanthanum strontium copper oxides (LSCO), both types having a transition temperature (Tc) less than liquid nitrogen (b.p. of 77 K), e.g., 35 K. Most notable among the HTS materials is yttrium barium copper oxide (YBCO), the first superconductor which achieved superconductivity at 91 K, well above the boiling point of nitrogen of 77 K. Since then, several other HTS materials have been discovered, including the bismuth strontium calcium copper oxides (BSCCO) with Tc up to 107 K, the thallium barium calcium copper oxides (TBCCO) with Tc up to 127 K, and the mercury thallium barium calcium copper oxides with Tc up to 138 K.
Methods for the preparation of films of HTS materials on various substrates are well known. These methods have been instrumental for converting HTS materials into tapes and wires, a necessary step in the effort for integrating these materials as wiring into conventional electrical grid systems and devices. In fact, HTS wires and tapes of significant length are produced by several companies.
The first HTS tapes suffered from unacceptably low critical current densities, a problem caused by poor alignment of grains in the HTS film or coating with grains of the substrate. Several techniques have therefore been developed to fabricate wires or tapes wherein grain alignment is produced. Of particular note is epitaxial growth of superconductors on the Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS). RABiTS substrates typically include a textured metal underlayer (for example, nickel or nickel alloy) and an epitaxial buffer layer (for example, Y2O3 and/or yttria-stabilized zirconia, YSZ). The development, preparation, and application of RABiTS is disclosed in several references and patents, including, for example, U.S. Pat. Nos. 7,087,113, 5,739,086, 5,741,377, 5,898,020, 5,958,599, and 5,944,966. Epitaxial superconductors on biaxially-textured substrates have significantly improved critical current densities of HTS tapes, and thus, improved utility for commercial applications.
However, HTS tapes and wires operating in the presence of alternating current (AC) suffer from a significant amount of energy dissipation, hereinafter referred to as “AC losses”. AC losses arise by several causes. A major contributor to AC loss is hysteretic energy loss in the superconducting oxide film caused by an oscillating external magnetic field. This loss contribution is proportional to the film width. Hence, it has been proposed to divide an FITS film into narrow filaments (i.e., by a filamentization process) to suppress. AC losses. However, the techniques currently known for imparting this filamentization tend to be cumbersome, complex, and expensive. Some examples of these types of techniques include physical scribing, laser scribing, photolithographic patterning, and ink-jet printing of HTS filaments directly onto a textured substrate using MOD solution precursors followed by HTS crystallization. See, for example, Barnes, P. N., et al., IEEE Transactions on Applied Superconductivity, Vol. 15, No. 2, June 2005; Daumling, M., et al., Studies of High Temperature Superconductors: AC loss in superconducting power cables (A. Narlikar, ed.), pp. 1-39, Nova Science Publishers (2000); Malozemoff, A. P., et al., Chinese Journal of Physics, vol. 34 (2-II), pp. 222-231 (April 1996); Gömöry, F., et al. Superconductor Science and Technology, vol. 17, S150-S154 (2004); U.S. Pat. No. 6,646,528, and U.S. Application Publication No. 2008/0113870.
There is also a significant interest in increasing the ability of the superconductor to carry supercurrents in the presence of high applied magnetic fields. This is known as increasing the “flux pinning”. HTS wires or conductors with good flux-pinning are needed for most large-scale applications in the electric power grid.